Light-emitting diodes (LEDs) belong to a class of the most efficient illumination light sources among currently available light sources. In particular, white LEDs find a rapidly expanding share in the market as the next-generation light source to replace incandescent lamps, fluorescent lamps, cold cathode fluorescent lamps (CCFL) for backlight, and halogen lamps. A white LED structure for use in light-emitting devices for illumination is constructed by combining a blue light-emitting diode (blue LED) with a phosphor capable of emitting light of longer wavelength, for example, yellow or green light upon blue light excitation, which technology is widely implemented on a commercial basis.
The mainstream of the white LED structure is a system in which a phosphor-mixed resin or glass is coated on an upper surface of a blue LED chip to encapsulate the LED chip so that the phosphor may convert the wavelength of part or all of blue light from the blue LED chip for producing pseudo-white light, i.e., integrated LED chip/phosphor system. There is also known a white LED structure based on another system in which a wavelength conversion member made of a phosphor-mixed resin or glass is separate from the encapsulant of a blue LED chip and disposed forward in the emission direction of the LED chip, so that the phosphor may convert the wavelength of part or all of blue light. The latter system is modified into an advanced system in which the phosphor-containing wavelength conversion member is spaced apart from a LED chip by a distance of several millimeters to several centimeters, for thereby achieving an improvement in emission efficiency and a suppression of color shift even when the LED chip is of high power so that properties of the phosphor are liable to degrade by the heat generated from the light-emitting portion. This system wherein the wavelength conversion member is spaced apart from the LED chip is known as “remote phosphor system,” on which active efforts are currently focused. In addition to the above advantages, the remote phosphor system has advantages as practical lighting fixtures including an improvement in overall color variation and a minimal variation during mass production.
The light-emitting device of the remote phosphor system has the structure wherein a wavelength conversion member, which is formed by dispersing yellow light-emitting phosphor particles, green light-emitting phosphor particles and optionally red light-emitting phosphor particles in a resin or glass, or by coating such phosphors to the surface of a transparent substrate, is disposed forward of a LED light source. Typical phosphors used in the wavelength conversion member of the remote phosphor system include Y3Al5O12:Ce3+ or cerium-activated yttrium-aluminum garnet phosphor represented by Y3Al5O12:Ce3+ (YAG phosphor), and Lu3Al5O12:Ce3+ or cerium-activated lutetium-aluminum garnet phosphor represented by Lu3Al5O12:Ce3+ (LuAG phosphor). Other phosphors include (Y,Gd)Al5O12:Ce3+, TbAl5O12:Ce3+, (Sr,Ca,Ba)2SiO4:Eu2+, and β-SiON:Eu2+. In some cases, phosphors such as CaSiN3:Eu2+ and Sr—CaSiN3:Eu2+ are used in combination with the foregoing phosphors for the purpose of improving color rendering.
Recently, for the light-emitting device using white LED, especially the lighting fixture using white LED, the color rendering of its light emission is considered important. In the illumination field which assumes that sunlight is ideal light or reference, light capable of representing a color close thereto is regarded as light having excellent color representation, i.e., color rendering. As the system for the numerical evaluation of color rendering, the system defined by Commission Internationale de l'Eclairage (CIE) in 1931 is widely used. In this system, differences in color rendition of eight color chips, numbered R1 to R8, are quantified on a scale of −100 to 100 and averaged to give an average color rendering index Ra. Also, in JIS Z 8726:1990 which expands the color rendering evaluation of the CIE system by adding seven color chips, numbered R9 to R15, to the eight color chips, the color rendering evaluation method using fifteen color chips is defined.
When the emission of white LED is evaluated by the above color rendering evaluation method, most prior art light-emitting devices have an average color rendering index Ra which is not regarded better than the existing illuminations such as conventional incandescent lamps and fluorescent lamps. They tend to have low values of the special color rendering index R9 using red color chip R9 among others.
The evaluation of color deviation of light is also important as the color rendering index of illuminating light. Natural light (sunlight) serving as the reference of emission color changes its tint from bluish white light to reddish light depending on the altitude of the sun, which is completely the same as the relationship of temperature and emission color of a red-hot object. Its chromaticity is drawn as a blackbody radiation locus on the xy chromaticity diagram (CIE 1931) as shown in FIG. 10.
The emission color of LED is not limited within the range of the blackbody radiation locus because its emission principle differs from thermal radiation like natural light. However, the light emission of LED whose chromaticity coordinates are spaced apart from the blackbody radiation locus gives an unnatural impression because the quality of light is perceived green or reddish purple even though the color reproduction of an object which is illuminated is satisfactory. Therefore, white LED is generally adjusted in emission color such that the chromaticity coordinates of emission lie on the blackbody radiation locus. That is, the LED light-emitting device for the illumination application is regarded excellent in color rendering when the color rendering index as an index of color reproduction (or color rendering), especially average color rendering index Ra, determined from color rendering indices R1 to R8, and special color rendering index R9 are high and the chromaticity coordinates of emission lie on the blackbody radiation locus.
A shift of the emission color of an LED light-emitting device from the blackbody radiation locus is known as color deviation and may be quantified as a deviation duv (Δuv) between illuminating light and the blackbody radiation locus in the (u,v) chromaticity diagram (CIE 1960). The (u,v) chromaticity diagram is used in determining a color deviation because the (u,v) chromaticity diagram is set such that a distance from an arbitrary point is equal to a perceivable color difference, and it is convenient for quantifying the color deviation. Among LED light-emitting devices, those having as small an absolute value of duv as possible are better, with those having an absolute value of duv equal to or less than 0.001 being preferred.
Furthermore, with respect to the color temperature (CCT) of illuminating light, light emission having a high color temperature, for example, of 6000K gives a high contrast cold impression. On the other hand, it is known that light having a low color temperature, for example, of 3000K gives a warm comfortable feeling. On use of a lighting fixture, an illumination having a color temperature complying with the environment is chosen.